Advances in interposer and 3D integration technologies are enabling ultra-compact multi-chip modules. However, silicon, which is often used as a packaging material for 3D integration and microelectromechanical systems (MEMS), often represents a large volume of a wafer-level-packaged module when compared against the volume occupied by active circuitry. In order to realize ultra-compact subsystems it is therefore desirable to embed non-active components, such as passive circuits, such as resistors, capacitors and inductors, and/or MEMS devices, in the unused volume of a silicon package.
Wafer-level packaging technology for integrating GaN-on-SiC Monolithic Microwave Integrated Circuits (MMICs) with integrated heat spreaders and through-wafer interconnects has been described in References [1], [2] and [3] below, which are incorporated herein by reference. A RF wafer-level-packaged subsystem with silicon-embedded passive components and CMOS or SiGe control chips is desirable; however, there are technological challenges associated with microfabricating features within the volume of a silicon wafer. These challenges include conformal coating of a photoresist mold, as described in References [4] and [5], which are incorporated herein by reference, metallic patterning inside deep cavities, as described in References [6], [7], [8] and [9], which are incorporated herein by reference, and process resolution.
Microfabricated inductors inside the volume of silicon wafers have previously been described in References [9] and [10] below, which are incorporated by reference; however, the described embedded inductors typically operate in the 1-200 MHz frequency range. Further, the inductors have wide (50-100 μm) conductors and wide (50-100 μm) interconductor gaps due to fabrication limitations. Inductors such as solenoid-type designs that use through-silicon vias to form vertical interconnects have been described in Reference [11] below, which is incorporated by reference. These inductors had a peak quality factor of 18.5 at 900 MHz.